As the quality of water from public water systems, particularly water from small public water systems, and private wells has become subject to contamination, the acceptability of tap water available to many has come to be questioned. Furthermore, whether the source of the water is from a well fed from an aquifer instead of from a river or reservoir the tap water may be characterized by problems associated with leaching of lead from plumbing, organic chemicals coming from industrial or agricultural runoff as well as some organic pollutants which ironically are by-products of water chlorination systems.
In addition, since public water systems are subject to breakdowns at treatment plants, microorganisms, and particularly pathogens may still be present in the tap water particularly when the source of water is surface water which has been contaminated by parasites such as cryptosporidium. It is generally recognized that few systems, be they public or private, perform the expensive filtering needed to remove such parasites. It is not uncommon for unsafe microorganisms levels to prompt a "boil order" so as to ensure that water is potable, particularly for those persons having compromised immunity.